Because LCD devices have the advantages of portability, low power consumption, and low radiation, they have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras, and the like. Furthermore, LCD devices are considered by many to have the potential to completely replace cathode ray tube (CRT) monitors and televisions.
FIG. 5 is an abbreviated circuit diagram of a typical LCD. The LCD 100 includes a first glass substrate (not shown), a second glass substrate (not shown) facing the first substrate, a liquid crystal layer (not shown) sandwiched between the first and second substrates, a scanning line driving circuit 11, a signal line driving circuit 12, and a timing control circuit 17.
The first substrate includes a number n (where n is a natural number) of scanning lines 13 that are parallel to each other and that each extend along a first direction, and a number k (where k is also a natural number) of signal lines 14 that are parallel to each other and that each extend along a second direction orthogonal to the first direction. The first substrate also includes a plurality of thin film transistors (TFTs) 15 that function as switching elements. The first substrate further includes a plurality of pixel electrodes 151 formed on a surface thereof facing the second substrate. Each TFT 15 is provided in the vicinity of a respective point of intersection of the scanning lines 13 and the signal lines 14.
Each TFT 15 includes a gate electrode, a source electrode, and a drain electrode. The gate electrode of the TFT 15 is connected to the corresponding scanning line 13. The source electrode of the TFT 15 is connected to the corresponding signal line 14. The drain electrode of the TFT 15 is connected to a corresponding pixel electrode 151.
The second substrate includes a plurality of common electrodes 152 opposite to the pixel electrodes 151. In particular, the common electrodes 152 are formed on a surface of the second substrate that faces the first substrate, and are made from a transparent material such as Indium-Tin Oxide (ITO) or the like. A pixel electrode 151, a common electrode 152 facing the pixel electrode 151, and liquid crystal molecules of the liquid crystal layer sandwiched between the two electrodes 151, 152 cooperatively define a single pixel unit.
The scanning lines 13 are connected to the scanning line driving circuit 11. The signal lines 14 are connected to the signal line driving circuit 12.
FIG. 6 is an abbreviated timing chart illustrating operation of the LCD 100. A scanning clock signal (CLK) is generated by the timing control circuit 17. Scanning signals G1-Gn are generated by the scanning line driving circuit 11, and are applied to the scanning lines 13. Gradation voltages (VD) are generated by the signal line driving circuit 12, and are sequentially applied to the signal lines 14. A common voltage (Vcom) is applied to all the common electrodes 152. Only one scanning signal pulse 19 is applied to each scanning line 13 during each single scan, the scanning signal pulse 19 having a duration which is equal to a period of the clock pulses of the scanning clock signal CLK. The scanning signal pulses 19 are output sequentially to the scanning lines 13.
The scanning line driving circuit 11 sequentially provides scanning pulses 19 (G1 to Gn) to the scanning lines 13, and activates the TFTs 15 respectively connected to the scanning lines 13. When the scanning lines 13 are thus scanned, the signal line driving circuit 12 outputs gradation voltages VD corresponding to the image data to the signal lines 14. Then the gradation voltages are applied to the pixel electrodes 151 via the activated TFTs 15. The potentials of all the common electrodes 152 are set at a uniform potential. The gradation voltages VD written to the pixel electrodes 151 are used to control the amount of light transmission at the corresponding pixel units. Consequently, the pixel units cooperatively provide an image for display on a screen of an LC panel 10 of the LCD 100.
The gradation voltage VD is a signal whose strength varies in accordance with each piece of image data, whereas the common voltage Vcom is a signal that has a constant value which does not vary at all.
If the LCD 100 provides motion picture display, problems of poor image quality may occur for a variety of reasons. For example, the residual image phenomenon may occur because a response speed of the liquid crystal molecules is too slow. In particular, when a gradation voltage variation occurs, the liquid crystal molecules are unable to track the gradation voltage variation within a single frame period, and instead produce a cumulative response during several frame periods. Consequently, considerable research is being conducted with a view to developing various high-speed response liquid crystal materials that can overcome this problem.
Further, the aforementioned problems such as the residual image phenomenon are not caused solely by the response speed of the liquid crystal molecules. For example, when the displayed image is changed in each frame period (the period that the scanning line driving circuit 11 completes sequential scanning from G1 to Gn once) to display the motion picture, the displayed image of one frame period may remain in a viewer's visual perception as an afterimage, and this afterimage overlaps with the viewer's perception of the displayed image of the next frame period. This means that from the viewpoint of a user, the image quality of the displayed image is impaired.
What is needed, therefore, is an LCD that can overcome the above-described deficiencies.